ospfd: Impl. per interface 'ip ospf area' command

ospfd: Impl. per interface 'ip ospf area' command

Use with interface command:
 interface ppp0
 ip ospf area 0.0.0.0
This will enable OSPF on ppp0 with area 0.0.0.0

Remove with "no ip ospf area"

* ospf_vty.c: add "ip ospf area (A.B.C.D|<0-4294967295>)" interface command

* ospfd.c: (ospf_interface_{un,}set) new helper function to enable/disable
  OSPF on a specific interface.
  (ospf_if_update) 2 possible paths now to deal with interface updates.

Acked-by: Donald Sharp <sharpd@cumulusnetworks.com>
[DL: this restores the tree to deccaf9...]